I think its dns, I've tried speaking to the tech support about it but they are useless. My findings:
C:\Documents and Settings\Dan>nslookup
www.google.co.uk
Server: cache1.ntli.net
Address: 194.168.4.100
*** cache1.ntli.net can't find
www.google.co.uk: Server failed
nslookup queries the DNS servers (nothing to do with the web caches) and this query appears to fail because ntl's DNS cache does not contain google.co.uk (which it should really).
I tore my hair out trying to explain this to the tech support, I hung up when they asked me to reboot for the 3rd time.
If we could get terrestrial or sky where we live I would drop NTL in a heart beat.